Printers and copier machines that operate with sheet media all require a structure to receive their finished sheet output. When the machines are to be capable of long runs, there often is provided a stacking apparatus (which can be either an accessory attachment or a machine subsystem) designed to receive output sheets and direct them into an orderly stack. Such stacker apparatus provides structure to guide the lead and side edges of the output sheet into the stack. A typical approach, for large quantity sheet output, is to provide an elevator platform which begins to receive sheets at a position proximate the sheet egress region of the output machine and gradually lowers. This enables the top sheets of the stack, which builds up on the platform, to remain at the proper sheet receiving location near the output machine's egress, so that the leading edge of newly output sheets can be guided into a proper position.
Because sheets do not have substantial rigidity, the trailing edge of the fed sheets cannot be depended upon to fall neatly into position on the stack top. For example, sheet curl, air cushion effects and side guide contacts can cause the sheets' trailing edge to remain upward from the stack top. A serious paper jam condition evolves if the trail end of the stack's top sheet remains sufficiently upward that it blocks the next output sheet.
The typical prior art approach for solving such trailing edge problems, has been to provide a continuously rotating roller with traction surface or flapper portions that direct the trailing edge of sheets downwardly onto the stack face. U.S. Pat. No. 4,056,264 discloses several configurations for implementing this approach. However, the provision of any continuously rotating impeller construction along the path from the output device to the stack necessarily creates a zone of constricted passage in the path and, thus, itself presents a potential for jamming.